1. Field of the Invention
The present disclosure generally relates to a transport device which transports a print medium, and more particularly to a transport device, an image forming device, a transport method, and a recording medium which are adapted to control a rotating speed of a transport roller unit which transports a print medium.
2. Description of the Related Art
In an image forming device, a transfer unit transfers a toner image formed on an intermediate transfer belt or a photoconductor drum, to a print medium. Subsequently, the toner image is fixed to the print medium by applying pressure and heat thereto. In the transfer unit, the print medium is pressed on the intermediate transfer belt or the photoconductor drum with a transfer roller. A transfer timing roller or a registration roller (upstream roller) is disposed at an upstream location of the transfer unit along a transport path.
When a size of a print medium is larger than a predetermined size, the print medium may be in an overlapping condition that it extends from a secondary transfer roller (downstream roller) to the upstream roller. The rotating speed of the downstream roller and the rotating speed of the upstream roller are controlled independently of each other. If the print medium is in an overlapping condition that it extends from the downstream roller to the upstream roller, tension in the print medium by an upstream roller of a fixing unit or compression in the print medium by the downstream roller may occur depending on a difference between the rotating speeds of these rollers. In the following, this phenomenon will be referred to as torque interference.
If torque interference between the two rollers occurs, either the upstream roller or the downstream roller slips, which causes deterioration of image quality or a color deviation. In particular, if a weight of unit area of a print medium is large (e.g., a cardboard sheet) and a peripheral speed of the upstream roller is larger than a peripheral speed of the downstream roller, the possibility that the downstream roller be made to slip by compression in the print medium by the downstream roller becomes high.
For example, Japanese Laid-Open Patent Publication No. 2008-158076 discloses an image forming device in which a print medium in a transport path in a fixing device is formed to have a loop amount. This image forming device is arranged to correct the rotating speed of a fixing roller of the fixing device at intervals of a predetermined time based on a result of comparison of a detected loop amount of the print medium and a proper loop amount.
Moreover, in the image forming device according to the related art, the force of a pair of upstream rollers to hold a print medium is reduced when the print medium is transported to reach the downstream roller, in order to cancel torque interference between the upstream rollers and the downstream roller.
According to this technique, even when a print medium is in an overlapping condition that it extends from the downstream roller to the upstream rollers, the print medium is not held by the upstream rollers. It is possible to prevent deterioration of image quality from occurring due to the torque interference.
However, in the image forming device of Japanese Laid-Open Patent Publication No. 2008-158076, the proper loop amount must be determined and stored beforehand. There is a problem that the amount of correction of the rotating speed of the fixing roller is dependent on the stored proper loop amount.
Usually, the amount of torque interference between the two rollers varies according to changes of the humidity of the environment on a daily basis and occasional changes of the image forming device. It is difficult to determine a proper loop amount beforehand. Hence, there is no guarantee that the amount of correction to the rotating speed of the fixing roller controlled as a result of the comparison between the detected loop amount and the loop amount is exact. In particular, when the print medium is a cardboard sheet, it is difficult to form a loop of the print medium in many cases.
In the image forming device according to the related art, which is arranged to reduce the force of the pair of upstream rollers to hold a print medium, there is a problem that the image forming device requires an actuator for adjusting the spacing of the upstream rollers, which increases the cost. In addition, it is difficult to modify the image forming device to have an additional space for installing the actuator.